One way to wean existing users towards linux or open source softwares is by giving them a feel of the various open source softwares under windows so that the migration towards linux becomes easier (training etc costs become less etc) A lot of equivalent open source softwares now exist in both worlds (linux and windows) such as gaim/openoffice/gimp etc etc.
I found these 2 sites which are very helpful http://www.opensourcewindows.org/ http://osswin.sourceforge.net/
Recently I came across mistika too which though not open source is a special purpose software under linux which shows that newer software development may be moving towards linux.
Kush